Lithium secondary battery and method of manufacturing the same

ABSTRACT

A lithium secondary battery includes a cathode formed from a cathode active material including a first cathode active material particle and a second cathode active material particle, an anode and a separation layer interposed between the cathode and the anode. The first cathode active material particle includes a lithium metal oxide including a concentration gradient and has a secondary particle structure formed from an assembly of primary particles. The second cathode active material particle includes a lithium metal oxide having a single particle structure. The first and second cathode active material particles each includes at least two metals except from lithium, and an amount of nickel is the largest among those of the metals in each of the first and second cathode active material particles.

CROSS-REFERENCE TO RELATED APPLICATION AND CLAIM OF PRIORITY

This application claims priority to Korean Patent Applications No.10-2018-0070610 filed on Jun. 20, 2018 in the Korean IntellectualProperty Office (KIPO), the entire disclosure of which is incorporatedby reference herein.

BACKGROUND 1. Field

The present invention relates to a lithium secondary battery and amethod of manufacturing the same. More particularly, the presentinvention relates to a lithium secondary battery including a lithiummetal oxide-based cathode active material and a method of manufacturingthe same.

2. Description of the Related Art

A secondary battery which can be charged and discharged repeatedly hasbeen widely employed as a power source of a mobile electronic devicesuch as a camcorder, a mobile phone, a laptop computer, etc., accordingto developments of information and display technologies. Recently, abattery pack including the secondary battery is being developed andapplied as an eco-friendly power source of an electric automobile suchas a hybrid vehicle.

The secondary battery includes, e.g., a lithium secondary battery, anickel-cadmium battery, a nickel-hydrogen battery, etc. The lithiumsecondary battery is highlighted due to high operational voltage andenergy density per unit weight, a high charging rate, a compactdimension, etc.

For example, the lithium secondary battery may include an electrodeassembly including a cathode, an anode and a separation layer, and anelectrolyte immersing the electrode assembly. The lithium secondarybattery may further include an outer case having, e.g., a pouch shape.

A lithium metal oxide may be used as a cathode active material of thelithium secondary battery, and high capacity, high power output and longlife-span may be preferably required in the cathode active material.However, as an application of the lithium secondary battery has beenexpanded, stability in a harsh condition such as high temperature or lowtemperature may be additionally needed in the lithium secondary battery.For example, when a penetration by an external object occurs through thelithium secondary battery, thermal stability for preventing ignition orshort-circuit may be needed in the lithium secondary battery or thecathode active material.

However, the cathode active material for implementing the aboveproperties may not be easily achieved. For example, Korean PublishedPatent Publication No. 10-2017-0093085 discloses a cathode activematerial including a transition metal compound and an ion adsorbingbinder, which may not provide sufficient life-span and stability.

SUMMARY

According to an aspect of the present invention, there is provided alithium secondary battery having improved operational stability andreliability.

According to an aspect of the present invention, there is provided amethod of manufacturing a lithium secondary battery having improvedoperational stability and reliability.

According to exemplary embodiments of the present invention, a lithiumsecondary battery includes a cathode formed from a cathode activematerial including a first cathode active material particle and a secondcathode active material particle, an anode and a separation layerinterposed between the cathode and the anode. The first cathode activematerial particle includes a lithium metal oxide including aconcentration gradient and has a secondary particle structure formedfrom an assembly of primary particles. The second cathode activematerial particle includes a lithium metal oxide having a singleparticle structure. The first and second cathode active materialparticles each includes at least two metals except from lithium, and anamount of nickel is the largest among those of the metals in each of thefirst and second cathode active material particles.

In some embodiments, the first cathode active material particle mayinclude a concentration gradient region between a core region and ashell region.

In some embodiments, the first cathode active material particle may havea continuous concentration gradient from a central portion to a surface.

In some embodiments, wherein the first cathode active material particlemay include a lithium metal oxide represented by the following ChemicalFormula 1.

Li_(x)M1_(a)M2_(b)M3_(c)O_(y)  [Chemical Formula 1]

In the Chemical Formula 1 above, M1 may be Ni, and M2 and M3 may beselected from Co, Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr,Nb, Mo, Al, Ga and B, and 0<x≤1.1, 2≤y≤2.02, 0.6≤a≤0.95, and0.05≤b+c≤0.4.

In some embodiments, 0.7≤a≤0.9 and 0.1≤b+c≤0.3 in the Chemical Formula1.

In some embodiments, M2 may be manganese (Mn) and M3 may be cobalt (Co).

In some embodiments, the second cathode active material particleincludes a lithium metal oxide represented by the following ChemicalFormula 2.

Li_(x)Ni_(a)CO_(b)Mn_(c)M4_(d)M5_(e)O_(y)  [Chemical Formula 2]

In the Chemical Formula 1 above, M4 may include at least one elementselected from Ti, Zr, Al, Mg, Si, B or Cr, and M5 may include at leastone element selected from Sr, Y, W or Mo. In the Chemical Formula 2,0<x<1.5, 2≤y≤2.02, 0.48≤a≤0.52, 0.18≤b≤0.22, 0.28≤c≤0.32, 0≤d≤0.25,0≤e≤0.15 and 0.98≤a+b+c≤1.02.

In some embodiments, 0.49≤a≤0.51, 0.19≤b≤0.21 and 0.29≤c≤0.31 in theChemical Formula 2.

In some embodiments, a mixing weight ratio of the first cathode activematerial particle and the second cathode active material particle may bein a range from 5:5 to 1:9.

In some embodiments, the second cathode active material particle mayhave a single crystalline structure.

In some embodiments, the second cathode active material particle mayhave a constant concentration from a central portion to a surface.

In some embodiments, an average diameter of the second cathode activematerial particle may be smaller than that of the first cathode activematerial particle.

In some embodiments, the number of the second cathode active materialparticles in a unit volume of the cathode may be greater than that ofthe first cathode active material particles.

In some embodiments, the second cathode active material particles mayserve as pore fillers between the first cathode active materialparticles.

In some embodiments, an amount of Ni in the second cathode activematerial particle may be smaller than that in the first cathode activematerial particle.

In some embodiments, each of the first and second cathode activematerial particles may further includes cobalt (Co) and manganese (Mn).An amount of Co and an amount of Mn in the second cathode activematerial particle may be each greater than that in the first cathodeactive material particle.

According to exemplary embodiments of the present invention as describedabove, a blend of a first cathode active material particle and a secondcathode active material particle may be used as a cathode activematerial. The first cathode active material particle may include aconcentration gradient and may have a secondary particle structure whichmay be formed from an aggregation of primary particles. The secondcathode active material particle may have a single particle structure ora single crystalline structure. For example, high capacity and highpower of the lithium secondary battery may be obtained from the firstcathode active material particle, and penetration stability and thermalstability of the lithium secondary battery may be obtained from thesecond cathode active material.

In some embodiments, the first and second cathode active materialparticles may each include a lithium metal compound having an excessamount of nickel (Ni). Thus, power output and capacity may be increasedfrom the cathode active material while preventing ignition or explosiondue to a drastic temperature increase by the second cathode activematerial particle having the single crystalline structure. Further,life-span and stability at high temperature of the lithium secondarybattery or the cathode may be additionally improved by the concentrationgradient of the first cathode active material particle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating a lithiumsecondary battery in accordance with exemplary embodiments;

FIG. 2 is a surface SEM (Scanning Electron Microscope) image of a secondcathode active material particle prepared by Example:

FIG. 3 is a cross-sectional SEM (Scanning Electron Microscope) image ofa second cathode active material particle prepared by Example; and

FIG. 4 is a Differential Scanning Calorimetry (DSC) graph of secondcathode active material particles prepared by Example and ComparativeExample.

DETAILED DESCRIPTION OF THE EMBODIMENTS

According to exemplary embodiments of the present inventive concepts, acathode active material including a first cathode active materialparticle which includes a concentration gradient and has a secondaryparticle structure formed from an aggregation of primary particles, anda second cathode active material particle having a single particlestructure is provided. According to exemplary embodiments, a lithiumsecondary battery including a cathode formed from the cathode activematerial is also provided.

Hereinafter, the present invention will be described in detail withreference to the accompanying drawings. However, those skilled in theart will appreciate that such embodiments described with reference tothe accompanying drawings are provided to further understand the spiritof the present invention and do not limit subject matters to beprotected as disclosed in the detailed description and appended claims.

The terms “first” and “second” are used herein to designate differentmembers or elements, and not to specify or limit an order of objects orthe number of elements.

FIG. 1 is a schematic cross-sectional view illustrating a lithiumsecondary battery in accordance with exemplary embodiments.

Referring to FIG. 1, a lithium secondary battery may include a cathode130, and anode 140 and a separation layer 150 interposed between thecathode 130 and the anode 140.

The cathode 130 may include a cathode current collector 110 and acathode active material layer 115 formed by coating a cathode activematerial on the cathode current collector 110.

In exemplary embodiments, the cathode active material may include afirst cathode active material particle and a second cathode activematerial particle. For example, the cathode active material may be ablend of the first cathode active material particle and the secondcathode active material particle.

The first cathode active material particle may include a concentrationgradient therein. For example, the first cathode active materialparticle may include a lithium metal oxide in which at least one metalforms a concentration gradient in the particle. The lithium metal oxidemay include nickel and other transition metal, and may include excessamount of nickel among metals except for lithium. The term “excessamount” used herein may indicate the largest amount or the largest molarratio among the metals except for lithium.

In some embodiments, the first cathode active material particle mayinclude a concentration gradient region between a central portion and asurface. For example, the first cathode material particle may include acore region and a shell region, and the concentration gradient regionmay be formed between the core region and the shell region. The coreregion and the shell region may each have a uniform or fixedconcentration.

In an embodiment, the concentration gradient region may be formed at thecentral portion. In an embodiment, the concentration gradient region maybe formed at the surface.

In some embodiments, the first cathode active material particle mayinclude the lithium metal oxide having a continuous concentrationgradient from the central portion of the particle to the surface of theparticle. For example, the first cathode active material particle mayhave a full concentration gradient (FCG) structure having asubstantially entire concentration gradient throughout the particle.

In some embodiments, concentrations of lithium and oxygen in the firstcathode active material particle may be substantially fixed or constant,and at least one element except for nickel and oxygen may have acontinuous concentration gradient from the central portion to thesurface or in the concentration gradient region.

The term “continuous concentration gradient” used herein may indicate aconcentration profile which may be changed with a uniform trend ortendency between the central portion and the surface portion. Theuniform trend may include an increasing trend or a decreasing trend.

The term “central portion” used herein may include a central point ofthe active material particle and may also include a region within apredetermined radius from the central point. For example, “centralportion” may encompass a region within a radius of about 0.1 μm from thecentral point of the active material particle.

The term “surface” or “surface portion” used herein may include anoutermost surface of the active material particle, and may also includea predetermined thickness from the outermost surface. For example,“surface” or “surface portion” may include a region within a thicknessof about 0.1 μm from the outermost surface of the active materialparticle.

In some embodiments, the continuous concentration particle may include alinear concentration profile or a curved concentration profile. In thecurved concentration profile, the concentration may change in a uniformtrend without any inflection point.

In an embodiment, at least one metal except for lithium included in thefirst cathode active material particle may have an increasing continuousconcentration gradient, and at least one metal except for lithiumincluded in the first cathode active material particle may have adecreasing continuous concentration gradient.

In an embodiment, at least one metal included in the first cathodeactive material particle except for lithium may have a substantiallyconstant concentration from the central portion to the surface.

In an embodiment, metals included in the first cathode active materialparticle except for lithium may include a first metal M1 and a secondmetal M2. The first metal M1 may have a continuously decreasingconcentration gradient from the central portion to the surface or in theconcentration gradient region. The second metal M2 have a continuouslyincreasing concentration gradient from the central portion to thesurface or in the concentration gradient region.

In an embodiment, the metals included in the first cathode activematerial particle except for lithium may further include a third metalM3. The third metal M3 may have a substantially constant concentrationfrom the central portion to the surface.

The term “concentration” used herein may indicate, e.g., a molar ratioof the first to third metals.

For example, the first cathode active material particle may include alithium metal oxide represented by the following Chemical Formula 1.

Li_(x)M1_(a)M2_(b)M3_(c)O_(y)  [Chemical Formula 1]

In the Chemical Formula 1 above, M1, M2 and M3 may be selected from Ni,Co, Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al,Ga and B, and 0<x≤1.1, 2≤y≤2.02, 0<a<1, 0<b<1, 0<c<1, and 0<a+b+c≤1.

In some embodiments, M1, M2 and M3 may be nickel (Ni), manganese (Mn)and cobalt (Co), respectively.

For example, nickel (Ni) may serve as a metal related to a capacity ofthe lithium secondary battery. As an amount of nickel becomes higher,capacity and power output of the lithium secondary battery may beimproved. However, an excessive amount of nickel may degrade of alife-span of the battery, and may be disadvantageous in an aspect ofmechanical and electrical stability of the battery. For example, whenthe amount of nickel is excessively increased, defects such as ignitionor short-circuit by a penetration of an external object may not besufficiently suppressed.

However, according to exemplary embodiments, nickel may be included asthe first metal M1. Thus, the amount of nickel at the central portionmay be relatively high to improve the capacity and power output of thelithium secondary battery, and a concentration of nickel may bedecreased from the central portion to the surface to prevent the defectsfrom the penetration and a life-span reduction.

For example, manganese (Mn) may serve as a metal related to themechanical and electrical stability of the lithium secondary battery. Inexemplary embodiments, an amount of Mn may be increased from the centralportion to the surface so that the defects from the penetration such asignition or short-circuit through the surface may be suppressed orreduced, and the life-span of the lithium secondary battery may be alsoenhanced.

For example, cobalt (Co) may serve as a metal related to a conductivityor a resistance of the lithium secondary battery. In exemplaryembodiments, a concentration of cobalt may be substantially fixed oruniformly maintained through an entire region of the first cathodeactive material particle. Thus, a current or a charge flow through thefirst cathode active material particle may be uniformly maintained whileimproving the conductivity of the battery and maintaining lowresistance.

In some embodiments, in Chemical Formula 1, the first metal M1 may benickel, and, e.g., 0.6≤a≤0.95 and 0.05≤b+c≤0.4. For example, aconcentration (or a molar ratio) of nickel may be continuously decreasedfrom about 0.95 to about 0.6.

If a lower limit of the nickel concentration (e.g., a surfaceconcentration) is less than about 0.6, capacity and power output at thesurface of the first cathode active material particle may be excessivelydeteriorated. If an upper limit of the nickel concentration (e.g., acentral concentration) exceeds about 0.95, life-span and mechanicalstability at the central portion may be excessively degraded.

Preferably, in a consideration of capacity and stability properties,0.7≤a≤0.9 and 0.1≤b+c≤0.3.

In an embodiment, an average concentration ratio Ni:Co:Mn throughout thefirst cathode active material particle may be adjusted as about 8:1:1.In this case, capacity and power output of the battery may be enhancedthrough Ni having a molar ratio of about 0.8, and conductivity andlife-span of the battery may be improved via Co and Mn havingsubstantially the same amount.

In some embodiments, the first cathode active material particle mayfurther include a coating on a surface thereof. For example, the coatingmay include Al, Ti, Ba, Zr, Si, B, Mg, P, an alloy thereof or on oxidethereof. These may be used alone or in a combination thereof. The firstcathode active material particle may be passivated by the coating sothat penetration stability and life-span of the battery may be furtherimproved.

In an embodiment, the elements, the alloy or the oxide of the coatingmay be inserted in the first cathode active material particle asdopants.

In some embodiments, the first cathode active material particle may beformed from a primary particle having a rod-type shape. An averagediameter of the first cathode active material particle (e.g., an averagediameter of a secondary particle) may be in a range from about 3 μm toabout 25 μm, preferably from about 7 μm to about 15 μm.

As described above, the lithium metal oxide having an excess amount ofNi may be used as the first cathode active material particle to achievehigh capacity/power output properties. Further, the concentrationgradient may be included in the first cathode active material particleso that reduction of life-span and operational stability due to theexcess amount of Ni may be prevented.

In exemplary embodiments, the first cathode active material particle mayhave a secondary particle shape formed from aggregation or assembly ofthe primary particles. For example, the concentration gradient of thefirst cathode active material particle may be created by aco-precipitation method while changing concentrations of metalprecursors as described below. Accordingly, concentrations of theprimary particles may be changed while being precipitated and aggregatedso that the secondary particle having the concentration gradientthroughout an entire particle may be obtained.

Thus, the structure of the first cathode active material particleincluding the concentration gradient may be easily achieved.

In exemplary embodiments, the second cathode active material particlemay have a single crystalline structure. The term “single crystallinestructure” used herein may indicate that the second cathode activematerial particle is a single particle or consists of a single particle.

For example, the second cathode active material particle maysubstantially consist of primary particles, and a secondary particlestructure formed from aggregation or assembly of the primary particlesmay be excluded. In some embodiments, the second cathode active materialparticle may include a structure in which a plurality of the primaryparticles may be integrally merged and converted into a substantiallysingle particle.

As described above, the first cathode active material particle may havethe secondary particle structure for easily forming the concentrationgradient. The secondary particle structure may include a plurality ofprimary particles therein, and thus cracks may be easily propagatedthrough the particle when a penetration of the battery occurs by theexternal object. Accordingly, heat generation or heat propagation may bedrastically accelerated in a short period due to an excess current whenshort-circuit between electrodes occurs by the penetration.

Further, when the cathode active material is coated on the cathodecurrent collector 110 and pressed for forming the cathode 130, pressuremay be transferred through pores between the primary particles in thefirst cathode active material particle to cause cracks and fractures ofthe first cathode active material particle. In this case, desiredcapacity and power output may not be obtained.

However, the second cathode active material particle having the singleparticle structure or the single crystalline structure may be usedtogether with the first cathode active material particle so that heatand shock propagation through cracks in the cathode active materialparticle may be reduced or prevented. Thus, life-span and long-termoperational reliability of the lithium secondary battery may beimproved. Further, drastic heat generation when the penetration occursmay be blocked to prevent ignition or explosion of the battery.

In example embodiments, the second cathode active material particle mayhave a substantially constant or fixed concentration throughout anentire region of the particle. The second cathode active materialparticle may include a lithium metal oxide.

In exemplary embodiments, the second cathode active material particlemay include a nickel-containing lithium metal oxide. In the secondcathode active material particle, a concentration of nickel may be lessthan that in the first cathode active material particle. In anembodiment, the concentration of nickel in the second cathode activematerial particle may be fixed to be less than the concentration ofnickel at the surface of the first cathode active material particle.

In some embodiments, the second cathode active material particle mayinclude at least two metals except for lithium. Concentrations of themetals except for lithium may be substantially uniform or constant froma central portion of the particle to a surface of the particle.

In some embodiments, the second cathode active material particle mayinclude nickel (Ni), cobalt (Co) and manganese (Mn). As described above,concentrations or molar ratios of Ni, Co and Mn may be substantiallyuniform or constant throughout the entire region of the second cathodeactive material particle.

In some embodiments, the second cathode active material particle mayinclude an excess amount of nickel, and the concentrations of nickel,manganese and cobalt may become sequentially smaller in consideration ofboth capacity and stability of the lithium secondary battery. In apreferable embodiment, the concentration ratio of Ni:Co:Mn in the secondcathode active material particle may be substantially about 5:2:3.

As described above, the second cathode active material particle may alsoinclude an excess amount of Ni, and may have a nickel concentration or anickel molar ratio less than that of the first cathode active materialparticle. Thermal stability and life-span of the lithium secondarybattery may be effectively added via a combination of the compositionand the single particle structure of the second cathode active materialparticle.

For example, the second cathode active material particle may include alithium metal oxide represented by the following Chemical Formula 2.

Li_(x)Ni_(a)Co_(b)Mn_(c)M4_(d)M5_(c)O_(y)  [Chemical Formula 2]

In the Chemical Formula 2 above, M4 may include at least one elementselected from Ti, Zr, Al, Mg, Si, B or Cr. M5 may include at least oneelement selected from Sr, Y, W or Mo. In Chemical Formula 2, 0<x<1.5,2≤y≤2.02, 0.48≤a≤0.52, 0.18≤b≤0.22, 0.28≤c≤0.32, 0≤d≤0.25, 0≤e≤0.15 and0.98≤a+b+c≤1.02. Preferably, 0.49≤a≤0.51, 0.19≤b≤0.21 and 0.29≤c≤0.31.

In some embodiments, an average diameter (D₅₀) of the second cathodeactive material particle may be in a range from about 2 μm to about 15μm. Within this range, life-span and stability of the lithium secondarybattery or the cathode 130 may be improved without hindering anelectrical activity of the first cathode active material particle by thesecond cathode active material particle.

In an embodiment, the average diameter (D₅₀) of the second cathodeactive material particle may be in a range from about 1 μm to about 10μm, preferably from about 1 μm to about 8 μm, more preferably from about2 μm to about 7 μm.

In exemplary embodiments, the average diameter of the second cathodeactive material particle may be less than that of the first cathodeactive material particle. For example, the average diameter of the firstcathode active material particle may be in a range from about 7 μm toabout 15 μm, and the average diameter of the second cathode activematerial particle may be in a range from about 2 μm to about 7 μm.

Accordingly, the second cathode active material particle may serve as apore filler. Thus, propagation of heat or cracks due to penetration orpressing may be avoided or reduced by the second cathode active materialparticle having the single particle structure or the single crystallinestructure. Additionally, heat diffusion in the first cathode activematerial particle when the penetration occurs may be blocked by thesecond cathode active material particle.

Spaces between the first cathode active material particles may be filledwith the second cathode active material particles having a relativelysmall average diameter so that an electrode density of the cathodeactive material layer 115 and an amount of the cathode active materialparticles may be increased relatively to an amount of a binder. Thus,capacity and power output from the cathode may be increased whileenhancing thermal and mechanical stability.

In exemplary embodiments, the number of the second cathode activematerial particles per unit volume of the cathode active material layer115 may be greater than that of the first cathode active materialparticles.

The first cathode active material particles and the second cathodeactive material particles may be each prepared and blended to obtain thecathode active material. A mixing weight ratio of the first cathodeactive material particle and the second cathode active material particlemay be from about 6:4 to about 1:9, preferably from about 5:5 to about1:9. Within this range, improvement of thermal stability and life-spanand prevention of ignition due to penetration may be easily implementedby the second cathode active material particles.

As described above, the first cathode active material particle may beprepared by the co-precipitation method. For example, metal precursorsolutions having different concentrations may be prepared. The metalprecursor solutions may include precursors of metals that may beincluded in the cathode active material. For example, the metalprecursors may include halides, hydroxides, acid salts, etc., of themetals.

For example, the metal precursors may include a nickel precursor, amanganese precursor and a cobalt precursor.

In exemplary embodiments, a first precursor solution having a targetcomposition at the central portion (e.g., concentrations of nickel,manganese and cobalt at the central portion) of the first cathode activematerial particle and a second precursor solution having a targetcomposition at the surface or the surface portion (e.g., concentrationsof nickel, manganese and cobalt at the surface) of the first cathodeactive material particle may be each prepared.

Subsequently, the first and second precursor solutions may be mixed andprecipitates may be formed. In some embodiments, a mixing ratio may becontinuously changed so that a continuous concentration gradient may beformed from the target composition at the central portion to the targetcomposition at the surface. Accordingly, the precipitate may include aconcentration gradient of the metals therein.

In some embodiments, a chelate agent and a basic agent (e.g., analkaline agent) may be added while forming the precipitate. In someembodiments, the precipitate may be thermally treated, and then alithium salt may be mixed and thermally treated again.

In some embodiments, the first cathode active material particle may beprepared by a solid state mixing/reaction, and the preparation methodmay not be limited to a solution-based process as described above.

In exemplary embodiments, the second cathode active material particlemay be prepared by a solid state thermal treatment of metal precursors.

For example, a lithium precursor (e.g., a lithium salt), the nickelprecursor, the manganese precursor and the cobalt precursor may be mixedaccording to the composition of the Chemical Formula 2 above to form aprecursor powder.

The precursor powder may be thermally treated in a furnace at, e.g., atemperature from about 700° C. to about 1200° C., and the precursors maybe merged or fused into a substantially single particle shape to obtainthe second cathode active material particle having a single crystallinestructure. The thermal treatment may be performed under an airatmosphere or an oxygen atmosphere so that the second cathode activematerial particle may be formed as a lithium metal oxide particle.

Within the above temperature range, generation of secondary particlesmay be substantially suppressed, and the second cathode active materialparticle without defects therein may be achieved. Preferably, thethermal treatment may be performed at a temperature from about 800° C.to about 1000° C.

The first cathode active material particle and the second cathode activematerial particle may be blended to form the cathode active material.The cathode active material may be mixed and stirred together with abinder, a conductive agent and/or a dispersive additive in a solvent toform a slurry. The slurry may be coated on the cathode current collector110, and pressed and dried to obtain the cathode 130.

During the pressing process, mechanical stability of the cathode activematerial may be maintained by the second cathode active materialparticle having the single particle or the single crystalline structure.

The cathode current collector 110 may include stainless-steel, nickel,aluminum, titanium, copper or an alloy thereof. Preferably, aluminum oran alloy thereof may be used.

The binder may include an organic based binder such as a polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP),polyvinylidenefluoride (PVDF), polyacrylonitrile,polymethylmethacrylate, etc., or an aqueous based binder such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC).

For example, a PVDF-based binder may be used as a cathode binder. Inthis case, an amount of the binder for forming the cathode activematerial layer 115, and an amount of the first and second cathode activematerial particles may be relatively increased. Thus, capacity and poweroutput of the lithium secondary battery may be further improved.

The conductive agent may be added to facilitate electron mobilitybetween the active material particles. For example, the conductive agentmay include a carbon-based material such as graphite, carbon black,graphene, carbon nanotube, etc., and/or a metal-based material such astin, tin oxide, titanium oxide, a perovskite material such as LaSrCoO₃or LaSrMnO₃.

In some embodiments, an electrode density of the cathode 130 may be in arange from about 3.0 g/cc to about 3.9 g/cc, preferably, from 3.2 g/ccto about 3.8 g/cc.

In exemplary embodiments, the anode 140 may include an anode currentcollector 120 and an anode active material layer 125 formed by coatingan anode active material on the anode current collector 120.

The anode active material may include a material that may be capable ofadsorbing and ejecting lithium ions. For example, a carbon-basedmaterial such as a crystalline carbon, an amorphous carbon, a carboncomplex or a carbon fiber, a lithium alloy, silicon, tin, etc., may beused. The amorphous carbon may include a hard carbon, cokes, amesocarbon microbead (MCMB) calcinated at a temperature of 1,500° C. orless, a mesophase pitch-based carbon fiber (MPCF), etc. The crystallinecarbon may include a graphite-based material, such as natural graphite,graphitized cokes, graphitized MCMB, graphitized MPCF, etc. The lithiumalloy may further include aluminum, zinc, bismuth, cadmium, antimony,silicon, lead, tin, gallium, or indium.

The anode current collector 120 may include gold, stainless-steel,nickel, aluminum, titanium, copper or an alloy thereof, preferably, mayinclude copper or a copper alloy.

In some embodiments, the anode active material may be mixed and stirredtogether with a binder, a conductive agent and/or a dispersive additivein a solvent to form a slurry. The slurry may be coated on the anodecurrent collector 120, and pressed and dried to obtain the anode 140.

The binder and the conductive agent substantially the same as or similarto those as mentioned above may be used. In some embodiments, the binderfor the anode 140 may include an aqueous binder such as such asstyrene-butadiene rubber (SBR) that may be used with a thickener such ascarboxymethyl cellulose (CMC) so that compatibility with thecarbon-based active material may be improved.

The separation layer 150 may be interposed between the cathode 130 andthe anode 140. The separation layer 150 may include a porous polymerfilm prepared from, e.g., a polyolefin-based polymer such as an ethylenehomopolymer, a propylene homopolymer, an ethylene/butene copolymer, anethylene/hexene copolymer, an ethylene/methacrylate copolymer, or thelike. The separation layer 150 may be also formed from a non-wovenfabric including a glass fiber with a high melting point, a polyethyleneterephthalate fiber, or the like.

In some embodiments, an area and/or a volume of the anode 140 (e.g., acontact area with the separation layer 150) may be greater than that ofthe cathode 130. Thus, lithium ions generated from the cathode 130 maybe easily transferred to the anode 140 without loss by, e.g.,precipitation or sedimentation. Therefore, the enhancement of power andstability by the combination of the first and second cathode activematerial particles may be effectively implemented.

In exemplary embodiments, an electrode cell 160 may be defined by thecathode 130, the anode 140 and the separation layer 150, and a pluralityof the electrode cells 160 may be stacked to form an electrode assemblyhaving, e.g., a jelly roll shape. For example, the electrode assemblymay be formed by winding, laminating or folding of the separation layer150.

The electrode assembly may be accommodated in an outer case 170 togetherwith an electrolyte to form the lithium secondary battery. In exemplaryembodiments, the electrolyte may include a non-aqueous electrolytesolution.

The non-aqueous electrolyte solution may include a lithium salt and anorganic solvent. The lithium salt may be represented by Li⁺X⁻, and ananion of the lithium salt X⁻ may include, e.g., F⁻, Cl⁻, Br⁻, I⁻, NO₃ ⁻,N(CN)₂ ⁻, BF₄ ⁻, ClO₄ ⁻, PF₆ ⁻, (CF₃)₂PF₄ ⁻, (CF₃)₃PF₃ ⁻, (CF₃)₄PF₂ ⁻,(CF₃)₅PF⁻, (CF₃)₆P⁻, CF₃SO₃ ⁻, CF₃CF₂SO₃ ⁻, (CF₃SO₂)₂N⁻, (FSO₂)₂N⁻,CF₃CF₂(CF₃)₂CO⁻, (CF₃SO₂)₂CH⁻, (SFi)₃C⁻, (CF₃SO₂)₃C⁻, CF₃(CF₂)₇SO₃ ⁻,CF₃CO₂ ⁻, CH₃CO₂ ⁻, SCN⁻, (CF₃CF₂SO₂)₂N⁻, etc.

The organic solvent may include propylene carbonate (PC), ethylenecarbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC),ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate,dimethyl sulfoxide, acetonitrile, dimethoxy ethane, diethoxy ethane,vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite,tetrahydrofuran, etc. These may be used alone or in a combinationthereof.

An electrode tab may be formed from each of the cathode currentcollector 110 and the anode current collector 120 to extend to one endof the outer case 170. The electrode tabs may be welded together withthe one end of the outer case 170 to form an electrode lead exposed atan outside of the outer case 170.

The lithium secondary battery may be fabricated into a cylindrical shapeusing a can, a prismatic shape, a pouch shape, a coin shape, etc.

Hereinafter, preferred embodiments are proposed to more concretelydescribe the present invention. However, the following examples are onlygiven for illustrating the present invention and those skilled in therelated art will obviously understand that various alterations andmodifications are possible within the scope and spirit of the presentinvention. Such alterations and modifications are duly included in theappended claims

Examples

(1) Preparation of First Cathode Active Material Particle

A total target composition was Li_(1.0)N_(10.8)Co_(0.1)Mn_(0.1)O₂, atarget composition at a core region wasLiNi_(0.84)2Co_(0.11)Mn_(0.05)O₂, and a target composition at a shellregion was Li_(1.0)Ni_(0.78)Co_(0.10)Mn_(0.12)O₂. A concentrationgradient region (decreasing Ni concentration and increasing Mnconcentration) was formed between the core region and the shell regionby continuously changing a mixing ratio of Ni and Mn precursors to formprecipitates and obtain the first cathode active material particle(hereinafter, abbreviated as NCM811).

(2) Preparation of Second Cathode Active Material Particle

Ni_(0.5)Co_(0.2)Mn_(0.3)(OH)₂ as an NCM precursor, and Li₂CO₃ and LiOHas lithium sources were grinded and mixed for 20 minutes. The mixedpower was fired at 1000° C. for 15 hours, and then grinding, sieving andde-ironing processes were performed to obtainLiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ (hereinafter, abbreviated as singlecrystalline NCM523) of a single crystalline type (see FIGS. 2 and 3).

(3) Fabrication of Secondary Battery

Blending ratios of the first and second cathode active materialparticles were adjusted as listed in following Table 1 to form cathodeactive materials. Denka Black was used as a conductive agent, and PVDFwas used as a binder. The cathode active material, the conductive agentand the binder were mixed by a weight ratio of 92:5:3 to form a cathodemixture. The cathode mixture was coated, dried, and pressed on analuminum substrate to form a cathode. A density of the cathode after thepressing was 3.5 g/cc or more.

An anode slurry was prepared by mixing 93 wt % of a natural graphite asan anode active material, 5 wt % of a flake type conductive agent KS6, 1wt % of SBR as a binder, and 1 wt % of CMC as a thickener. The anodeslurry was coated, dried, and pressed on a copper substrate to form ananode.

The cathode and the anode obtained as described above were notched witha proper size and stacked, and a separator (polyethylene, thickness: 25μm) was interposed between the cathode and the anode to form anelectrode cell. Each tab portion of the cathode and the anode waswelded. The welded cathode/separator/anode assembly was inserted in apouch, and three sides of the pouch (e.g., except for an electrolyteinjection side) were sealed. The tab portions were also included insealed portions. An electrolyte was injected through the electrolyteinjection side, and then the electrolyte injection side was also sealed.Subsequently, the above structure was impregnated for more than 12hours.

The electrolyte was prepared by dissolving 1M LiPF₆ in a mixed solventof EC/EMC/DEC (25/45/30; volume ratio), and then 1 wt % of vinylenecarbonate, 0.5 wt % of 1,3-propensultone (PRS), and 0.5 wt %/o oflithium bis(oxalato) borate (LiBOB) were added.

Comparative Examples

Ni_(0.5)Co_(0.2)Mn_(0.3)(OH)₂ and Li₂CO₃ were mixed in a solid state toprepare LiNi_(0.5)Co_(0.2)Mn_(0.3)O₂ which had a secondary particlestructure formed from an aggregation of primary particles (hereinafter,abbreviated as a multi-crystalline NCM523) as a second cathode activematerial particle.

Secondary batteries of Comparative Examples were fabricated by the samemethod as that of Examples except that the multi-crystalline NCM523 wasused as the second cathode active material particle and cathode activematerials having weight ratios as listed in Table 1 below were used.

Experimental Example

(1) Evaluation on Life-Span Property at Room Temperature

500 cycles of a charging (CC-CV 1.0 C 4.2V 0.05 C CUT-OFF) and adischarging (CC 1.0 C 2.7V CUT-OFF) were repeated using the secondarybatteries of Examples and Comparative Examples. A life-span property wasmeasured by a percentage (%) of a discharging capacity at a 500th cyclewith respect to that at a first cycle.

(2) Evaluation on Penetration Stability

Secondary batteries of Examples and Comparative Examples were charged 1C 4.2V 0.1 C CUT-OFF), and then penetrated from an outside of thebatteries by a nail having a diameter of 3 mm at a rate of 80 mm/sec. Apenetration stability was evaluated based on a standard below.

<Penetration Stability, EUCAR Hazard Level>

L1: No malfunction occurs from the battery

L2: Irreversible damages of the battery occur

L3: A weight of an electrolyte in the battery was decreased by a ratioless than 50%

L4: A weight of an electrolyte in the battery was decreased by a ratioof 50% or more

L5: Ignition or explosion occurs

The results are shown in Table 1 below.

TABLE 1 Second First Cathode Cathode Active Active Material MixingLife-span Pene- Material Particle Weight (%) tration Particle (D50: 7μm) Ratio (500cycle) Stability Example 1 NCM811 single 90:10 90.1% L5crystalline NCM523 Example 2 NCM811 single 80:20 90.9% L4 crystallineNCM523 Example 3 NCM811 single 70:30 92.8% L3 crystalline NCM523 Example4 NCM811 single 60:40 94.1% L3 crystalline NCM523 Example 5 NCM811single 50:50 95.3% L3 crystalline NCM523 Example 6 NCM811 single 40:6096.1% L3 crystalline NCM523 Example 7 NCM811 single 30:70 97.6% L3crystalline NCM523 Example 8 NCM811 single 20:80 99.3% L3 crystallineNCM523 Example 9 NCM811 single 10:90 99.5% L3 crystalline NCM523Comparative NCM811 multi 90:10 85.8% L5 Example 1 crystalline NCM523Comparative NCM811 multi 80:20 87.1% L5 Example 2 crystalline NCM523Comparative NCM811 multi 70:30 88.6% L5 Example 3 crystalline NCM523Comparative NCM811 multi 60:40 89.7% L5 Example 4 crystalline NCM523Comparative NCM811 multi 50:50 90.7% L4 Example 5 crystalline NCM523Comparative NCM811 multi 40:60 92.3% L4 Example 6 crystalline NCM523Comparative NCM811 multi 30:70 93.5% L3 Example 7 crystalline NCM523Comparative NCM811 multi 20:80 94.7% L3 Example 8 crystalline NCM523Comparative NCM811 multi 10:90 98.1% L3 Example 9 crystalline NCM523Comparative NCM811 multi — 98.2% L3 Example 10 crystalline NCM523

Referring to Table 1 above, the batteries of Examples using the singlecrystalline NCM523 together with NCM811 having a concentration gradientshowed improved life-span and penetration stability compared to thebatteries of Comparative Examples using the multi crystalline NCM523.The batteries of Examples 5-9 including 50% or more of the singlecrystalline NCM523 showed life-span retentions of 95% or more, and moreimproved penetration stability.

(3) Evaluation on DSC

Heating values according to a temperature change of the cathode activematerial particle in Examples (the single crystalline NCM523) and thecathode active material particle in Comparative Examples (the multicrystalline NCM523) were measured using a DSC (Differential ScanningCalorimetry) method to evaluate thermal properties of the second cathodeactive material particles.

FIG. 4 is a Differential Scanning Calorimetry (DSC) graph of secondcathode active material particles prepared by Example and ComparativeExample.

Referring to FIG. 4, a heating peak occurred at a temperature greaterthan 330° C. in the cathode active material particle of Example. Aheating peak occurred at a temperature greater than 320° C. in thecathode active material particle of Comparative Example.

(4) Overcharging Test

The secondary batteries of Example 8 and Comparative Example 8 includingthe first cathode active material particle and the second cathode activematerial particle by a mixing ratio of 20:80 were charged from a stateof SOC 0% to a state of SOC 100% by a charging current 6V for 2.5 hours,and thermal stability was evaluated using EUCAR Hazard Level standard.

After the test above, the secondary battery of Example 9 was not ignited(L3 level). However, a weight of the electrolyte in the secondarybattery of Comparative Example 8 was drastically decreased to L4 level.

What is claimed is:
 1. A lithium secondary battery, comprising: acathode formed from a cathode active material including a first cathodeactive material particle and a second cathode active material particle;an anode; and a separation layer interposed between the cathode and theanode, wherein the first cathode active material particle includes alithium metal oxide including a concentration gradient and has asecondary particle structure formed from an assembly of primaryparticles, wherein the second cathode active material particle includesa lithium metal oxide having a single particle structure, wherein thefirst and second cathode active material particles each includes atleast two metals except from lithium, and an amount of nickel is thelargest among those of the metals in each of the first and secondcathode active material particles.
 2. The lithium secondary batteryaccording to claim 1, wherein the first cathode active material particleincludes a concentration gradient region between a core region and ashell region.
 3. The lithium secondary battery according to claim 1,wherein the first cathode active material particle has a continuousconcentration gradient from a central portion to a surface.
 4. Thelithium secondary battery according to claim 1, wherein the firstcathode active material particle includes a lithium metal oxiderepresented by the following Chemical Formula 1:Li_(x)M1_(a)M2_(b)M3_(c)O_(y)  [Chemical Formula 1] wherein, in theChemical Formula 1 above, M1 is Ni, and M2 and M3 are selected from Co,Mn, Na, Mg, Ca, Ti, V, Cr, Cu, Zn, Ge, Sr, Ag, Ba, Zr, Nb, Mo, Al, Gaand B, and 0<x≤1.1, 2≤y≤2.02, 0.6≤a≤0.95, and 0.05≤b+c≤0.4.
 5. Thelithium secondary battery according to claim 4, wherein, 0.7≤a≤0.9 and0.1≤b+c≤0.3 in the Chemical Formula
 1. 6. The lithium secondary batteryaccording to claim 4, wherein M2 is manganese (Mn) and M3 is cobalt(Co).
 7. The lithium secondary battery according to claim 1, wherein thesecond cathode active material particle includes a lithium metal oxiderepresented by the following Chemical Formula 2:Li_(x)Ni_(a)Co_(b)Mn_(c)M4_(d)M5_(c)O_(y)  [Chemical Formula 2] wherein,in the Chemical Formula 1 above, M4 includes at least one elementselected from Ti, Zr, Al, Mg, Si, B or Cr, and M5 includes at least oneelement selected from Sr, Y, W or Mo, wherein, in the Chemical Formula2, 0≤x≤1.5, 2≤y≤2.02, 0.48≤a≤0.52, 0.18≤b≤0.22, 0.28≤c≤0.32, 0≤d≤0.25,0≤e≤0.15 and 0.98≤a+b+c≤1.02.
 8. The lithium secondary battery accordingto claim 7, wherein 0.49≤a≤0.51, 0.19≤b≤0.21 and 0.29≤c≤0.31 in theChemical Formula
 2. 9. The lithium secondary battery according to claim1, wherein a mixing weight ratio of the first cathode active materialparticle and the second cathode active material particle is in a rangefrom 5:5 to 1:9.
 10. The lithium secondary battery according to claim 1,wherein the second cathode active material particle has a singlecrystalline structure.
 11. The lithium secondary battery according toclaim 1, wherein the second cathode active material particle has aconstant concentration from a central portion to a surface.
 12. Thelithium secondary battery according to claim 1, wherein an averagediameter of the second cathode active material particle is smaller thanthat of the first cathode active material particle.
 13. The lithiumsecondary battery according to claim 12, wherein the number of thesecond cathode active material particles in a unit volume of the cathodeis greater than that of the first cathode active material particles. 14.The lithium secondary battery according to claim 12, wherein the secondcathode active material particles serve as pore fillers between thefirst cathode active material particles.
 15. The lithium secondarybattery according to claim 1, wherein an amount of Ni in the secondcathode active material particle is smaller than that in the firstcathode active material particle.
 16. The lithium secondary batteryaccording to claim 1, wherein each of the first and second cathodeactive material particles further includes cobalt (Co) and manganese(Mn), wherein an amount of Co and an amount of Mn in the second cathodeactive material particle are each greater than that in the first cathodeactive material particle.